Despite significant advances in liquid crystal display (LCD) technology, resulting in very high performance displays with improved metrics such as display area, brightness, image contrast, resolution, color gamut, bit-depth, response time and wide view performance, color shift with viewing angle remains a problem for many types of LCD.
In order to improve the wide-view performance of LCDs, several technologies have been developed. Displays have been produced with angular compensation films such as the splayed-discotic Wide-View film for Twisted Nematic (TN) displays, multidomained pixels for Vertically Aligned Nematic (VAN) and In-Plane Switching (IPS) mode displays, and improved electrode geometries. These developments have enabled displays with no contrast inversion problem at wide viewing angles, i.e. although the absolute brightness of a pixel may change with viewing angle, a pixel which is switched to have an on-axis brightness higher than another pixel will remain brighter at all viewing angles, and vice versa. However, the amount of variation in brightness of a pixel with viewing angle is still a function of the on-axis brightness of the pixel in most types of LCD. This has the effect that in a color display comprising an array of pixels, each of which is composed of a plurality of color sub-pixels, such as red, green and blue sub-pixels in an RGB stripe display for example, if the pixel is displaying a color consisting of different brightness values of the three color components, these different brightness values can shift by a different amount with viewing angle, resulting in a shift in the perceived color.
Again, several technologies have been developed to mitigate this effect. The most effective of these utilise a split sub-pixel architecture, whereby each color sub-pixel in the display consists of two or more regions. In order to produce a given brightness overall to the viewer positioned along the normal to the plane of the display (on-axis), these are made to produce individually a different brightness, one brighter than the other, such that the average brightness of the two regions on-axis is the desired overall brightness, and the shift in brightness with viewing angle of each portion is different so the averaged shift of the two combined is less pronounced than each taken individually.
This method is known as partial spatial dither or digital halftoning, and can be implemented using a capacitive potential divider between the regions of the split sub-pixel, as described in U.S. Pat. No. 4,840,460, and US 20050219186A1, or it can be implemented by using an additional source line per color sub-pixel, such that each of the two regions of the sub pixel receives an independently controlled signal voltage when they are activated by a common gate line. This second implementation is described in U.S. Pat. No. 6,067,063, and the two general approaches are summarized, and optimised relationships between the voltages applied to the brighter and darker regions of the sub-pixel for reduced color shift given in U.S. Pat. No. 7,079,214.
It is not necessary to have a split sub-pixel architecture to implement such a method. The technique can effectively be implemented in software, or in the LCD control electronics, and applied to any existing color display by adjusting the brightness of whole color sub-pixels up and down alternately, either in the spatial or temporal domain, to create the same effect at the expense of the effective resolution of the display. Brightness is effectively transferred between the color components of neighbouring pixels, so that no overall change occurs, but the difference in brightness of neighbouring pixels is increased, resulting in an average shift in brightness with viewing angle which is reduced. This is described in U.S. Pat. No. 6,801,220 and U.S. Pat. No. 5,847,688. In U.S. Pat. No. 6,801,220, this is implemented by an image processing method in which the image data input to the LCD is manipulated by means of a Look-Up Table (LUT), so that for each input data level, a pair of output data levels is provided which, when displayed by neighbouring pixels on the LCD, are averaged by the eye of the viewer (assuming sufficient display resolution and viewing distance) to appear the same as if the original input data level were displayed on both pixels. The image processing method therefore alternates spatially across the display which of the pair of output data values is applied to each pixel for a given input data value.
All of the above methods implement a halftoning method, either within each color sub-pixel of the display in the case of the split sub-pixel, or within groups of neighbouring sub-pixels in the case of the image processing methods, in which the relationship between the brightness of the sub-pixels or sub-pixel regions which are combining to provide the required average brightness is fixed, either by the ratio of the capacitive potential divider applied between the regions, or by the use of a single LUT to output the brighter and darker data levels for each input data levels for all pixels in the display.
As a result of this fixed relationship, both of the above approaches in pixel hardware and display software or control electronics suffer from the limitation that in order to optimally reduce the color shift with viewing angle of the display, the effective pixel brightness observed by the on-axis viewer has to be composed of two or more regions of different brightness, for all but the off state (zero voltage applied to all regions). Both regions, either the plurality of regions within a split color sub-pixel, or neighbouring whole color sub-pixels which have been subject to a transfer of luminance within local groups, therefore cannot be fully bright without compromising the effectiveness of the method in reducing the color shift.
An LCD display generally consists of several component parts including:
1. A backlighting unit to supply even, wide angle illumination to the panel.
2. Control electronics to receive digital image data and output analogue signal voltages for each pixel, as well as timing pulses and a common voltage for the counter electrode of all pixels. A schematic of the standard layout of an LCD control electronics is shown in FIG. 1 (see, E. Lueder, Liquid Crystal Displays, Wiley and Sons Ltd., 2001).3. A liquid crystal (LC) panel, for displaying an image by spatial light modulation, includes two opposing glass substrates, onto one of which is disposed an array of pixel electrodes and an active matrix array to direct the electronic signals, received from the control electronics, to the pixel electrodes. Onto the other substrate is usually disposed a uniform common electrode and color filter array film. Between the glass substrates is contained a liquid crystal layer of given thickness, usually 2-6 μm, which may be aligned by the presence of an alignment layer on the inner surfaces of the glass substrates. The glass substrates will generally be placed between crossed polarising films and other optical compensation films to cause the electrically induced alignment changes within each pixel region of the LC layer to produce the desired optical modulation of light from the backlight unit and ambient surroundings, and thereby generate the image.
Generally the LCD Control Electronics (referred to herein also as control electronics) will be configured specifically to the electro-optical characteristics of the LC panel so as to output signal voltages which are dependent on the input image data in such a way as to optimise the perceived quality of the displayed image, i.e. resolution, contrast, brightness, response time, etc., for the principal viewer, observing from a direction normal to the display surface (on-axis). The relationship between the input image data value for a given pixel and the observed luminance resulting from the display (gamma curve) is determined by the combined effect of the data-value to signal voltage mapping of the display driver, and the signal voltage to luminance response of the LC panel.
The LC panel will generally be configured with multiple LC domains per pixel and/or passive optical compensation films so as to preserve the display gamma curve as closely as possible to the on-axis response for all viewing angles, thereby providing substantially the same high quality image to a wide viewing region. However, it is the inherent property of liquid crystal displays that their electro-optic response is angularly dependent and the off-axis gamma curve will differ from the on-axis one, and while contrast inversion problems have largely been solved with multidomain pixels and improved compensation films, color shift with angle remains a problem.
For reasons of clarity, the following examples to illustrate this effect and descriptions of the embodiments to reduce it will be directed toward VAN mode LCD displays, with 8 bit per color gradation control. The problem of color shift with angle is not restricted to VAN mode displays or displays of any particular color depth, nor is the applicability of the embodiments described herein, so this should not detract from the scope of the invention, which is applicable to any LCD which exhibits color shift with angle.
FIG. 2 shows the measured angular dependence of the luminance of a multidomained VAN mode LCD in a mobile phone display, at shades of grey from input data level=0 (black) to 255 (white) in steps of 32. FIG. 3(a) shows the points of FIG. 2 at 0° and 50° inclination to the right hand side (horizontal in the orientation in which the display is normally observed) plotted against the input data level. The On-Axis curve is known as the display “gamma” curve, being designed to approximately follow the relationship
      L          L      max        =            (              D                  D          max                    )        γ  where L is the output luminance, for a given data level D, and γ (gamma) is the power relating the two when each is normalized to their maximum value. The gamma value is typically engineered to be in the region of 2.0 to 2.4, and is approximately 2.3 for the display shown in FIGS. 2 and 3.
FIG. 3(b) shows the brightness of the display at 50° inclination as a function of the brightness on-axis, both normalized to their maximum values.
From the figures it can clearly be seen that the typical behaviour for a VAN mode display is for mid-grey levels to appear disproportionately bright when viewed off-axis. This is further illustrated in FIG. 4, which shows the luminance as a function of viewing angle, normalized to the luminance of the data=255 state at each angle, for the same VAN mode display displaying input data=255, 160 and zero. From this figure, it can be seen that if a pixel was input with data=255 to the red color sub-pixel, with data=160 to the green color sub-pixel and with data=0 to the blue color sub-pixel, on-axis, the ratio of normalized luminances is approximately 1:0.35:0 for R:G:B, which would result in an orange colored appearance for the pixel. However, when viewed from 50° inclination, the ratio of color components is approximately 1:0.77:0.03, which would result in a yellow appearance for the pixel. This is the cause of the color-shift with viewing angle, and it can be seen that, for VAN mode displays in particular, the degree of color shift is greatest for colors which are composed of one color component near maximum luminance, and one or two color components in the mid-luminance range.
The aim of conventional digital halftoning methods is to reduce this change in relative brightness of the color components of a pixel by replacing sub-pixels which are displaying 50% of maximum luminance with a half sub-pixel region at maximum luminance, and a half sub-pixel region at minimum luminance, in the case of the hardware method, or replace a neighbouring pair of sub-pixels which are set to display 50% of maximum luminance with one at maximum luminance and one at minimum luminance in the case of the software or control electronics methods. A mid-luminance sub-pixel or sub-pixel pair thereby becomes effectively a maximum luminance sub-pixel of half the standard emitting area, so the luminance of the sub-pixel or sub-pixel pair is half that of the maximum luminance state at all viewing angles, so color shift is avoided.
Obviously, only a pair of sub-pixels at exactly the average of maximum and minimum luminance can be replaced with one at maximum and one at minimum luminance without affecting the combined appearance of the pair to the on-axis viewer. Pixels with other values can be replaced by one pixel at minimum or maximum luminance, and the other at the some luminance to make up the required overall average. For this reason, U.S. Pat. No. 6,801,220 provides the LUT illustrated in FIG. 5(a) to relate pairs of pixels with a maximal difference between the pixels in the pair and an average luminance equal to the average luminance of two pixels of the same input data level, for all input data levels on a display with a gamma equal to 2.2.
The equivalent of FIG. 3(b) for a display in which the pixel data values have been altered according to the LUT of FIG. 5(a), is shown in FIG. 5(b). As can be seen from this figure, the normalized luminance at 50° inclination now no longer differs from the normalized luminance on-axis for pixels at 50% of maximum luminance. Color pixels comprising combinations of color components at minimum, 50% and maximum luminance will now have no color shift with viewing angle. The normalized luminance at 50° inclination does not coincide with the normalized luminance on-axis for pixels set to display luminance values other than these however, particularly for pixels set to display 25% or 75% of maximum luminance, so when displaying colors with one or more color components at these levels, color shift will still be apparent. Also, as the reduction in color shift is significantly greater using the LUT method described above for pixels with a color component at 50% luminance on-axis then the same pixel with the same color component moved to 75% luminance, images which have smoothly varying color across the display e.g. one color component changing from 50% to 75% luminance, will not appear to vary smoothly off axis, as not only is the color changing across the display, the degree of correction of color shift also changes, producing an exaggerated effect which can be very off-putting to the viewer. In order to resolve this problem, U.S. Pat. No. 6,801,220 suggests a modified LUT in which pairs of pixels with the same input data level are replaced with one higher and one lower data level pixel, but with the difference in the adjusted pixels no longer maximised. This will reduce the effectiveness of the color shift reduction effect however.
For these reasons, in many LCD television displays, where accurate picture reproduction over a very wide range of viewing angles is an important feature, all input data levels, except for data=0, are displayed using a split sub-pixel with different brightness on each sub-pixel half. This allows all colors except black to be composed of color components consisting of two different brightness regions, and consequently two different viewing angle variations which are averaged to produce a more uniform response. Color shift is thereby reduced; the maximum transmission (brightness) of the display is also consequently reduced.
FIG. 6(a) shows the measured luminance of the two halves of a split sub-pixel in a commercially available VAN mode LCD television. As can be seen in the Figure, the darker sub-pixel half reaches approximately 65% of the luminance of the brighter sub-pixel half at input data=255. This results in the display having a brightness of 82.5% of its maximum in order to preserve colors at wide viewing angles. FIG. 6(b) shows the corresponding normalized luminance at a viewing angle of 50° inclination against the normalized on-axis luminance for the television as measured. As can be seen, the off-axis luminance is still not completely linear with on-axis luminance, so color will still shift, but less than an unmodified display, and more uniformly with input data level than the result of the LUT method of FIG. 5, so the exaggerated off-axis color changes associated with that method do not occur.
It is therefore clear that a requirement exists for an optimised method of reducing the color shift with viewing angle in LCD displays which provides the required degree of color shift reduction, with minimum loss of peak brightness of the display.